itch.io

If you click the gear icon next to the Launch button, you can then click manage, and then click Check for Update. Otherwise you can uninstall and reinstall from the same menu

Definitely not! One purchase gets you all patches.
I recommend using the Itch App, as it updates your games automatically (or with little effort). Without the app, just log in and find the game in your library to get the latest version.

If you are using a browser, scroll to the top of the page and click download, and it will take you to a page to download the latest patch.